The invention concerns ready-to-use metal wires and methods for obtaining said wires. These ready-to-use wires are utilized, for example, to reinforce plastic or rubber articles, and in particular pipes, belts, plys and pneumatic tires.
The term "ready-to-use wire" as employed in this application means, in a manner known in the field, that this wire can be used for the proposed application without subjecting it to a heat treatment that could modify its metallurgical structure, and without subjecting it to deformation of its metal substance, for example, to a drawing process that can modify its diameter.
Patent application WO-A-92/14811 describes a method for obtaining ready-to-use wire comprising a steel substrate whose structure involves more than 90% cold-hammered annealed martensite, the steel having a carbon content of not less than 0.05% and not more than 0.6%, this substrate being coated with a metal alloy other than steel, for instance a brass alloy. The method for obtaining this wire includes a hardening treatment on a cold-hammered wire, involving heating the wire above transformation point AC3 to give it a homogeneous austenitic structure and then quick-cooling it at the rate of at least 150.degree. C./second, below the end point of the martensitic transformation. After this hardening treatment, at least two metals are deposited on the wire, and the wire is heated to stimulate by diffusion the formation of an alloy of these two metals, generally brass. The wire is then cooled and cold-hammered. The method described in this document includes the following specific advantages:
1. the use of a starting wire rod with a carbon content less than that of perlitic steel; PA1 2. great flexibility in the choice of wire rod diameters and of the ready-to-use wire thus obtained; PA1 3. drawing done starting with the wire rod at high speeds and with fewer breaks; PA1 4. the diffusion treatment is done at the time the wire is annealed, which holds down production costs. PA1 a) The annealing temperature necessary to achieve good diffusion of the coating does not always correspond precisely to the temperature necessary to obtain sufficient strength prior to drawing. PA1 b) The mechanical properties obtained after annealing vary rapidly in terms of the temperature variation introduced following the inevitable dispersion of the heating systems. PA1 c) The hardenability of the steel is insufficient; in other words, it is necessary to cool it at high speed in order to obtain a structure that is totally or almost totally martensitic. If the cooling speed is too slow, phases other than martensite can appear, such as bainite, for example. This high hardening speed is a major manufacturing constraint. PA1 a) It comprises a microalloyed steel with a carbon content of not less than 0.2% by weight and not more than 0.6% by weight; the steel also contains at least one alloy element chosen from the group consisting of vanadium, molybdenum and chromium, the steel containing not less than 0.08% and not more than 0.5% by weight of the alloy element or of all the alloy elements combined; PA1 b) The steel has a structure consisting almost entirely of cold-hammered annealed martensite; PA1 c) The wire diameter is not less than 0.10 mm and not more than 0.50 mm; PA1 d) The wire rupture strength is not less than 2800 Mpa. PA1 a) It starts with a steel wire rod; the steel has a carbon content of not less than 0.2% by weight and not more than 0.6% by weight; the steel also contains at least one alloy element chosen from the group comprised of vanadium, molybdenum and chromium, with steel comprising not less than 0.08% and not more than 0.5% by weight of the alloy component or of all the alloy components combined; PA1 b) The wire rod is deformed so that the diameter of the wire after such deformation is less than 3 mm; PA1 c) The deformation is stopped, and the deformed wire undergoes a hardening heat treatment; this treatment consists in heating the wire to above the point of transformation AC3 to give it a homogeneous austenitic structure, then cooling it at least practically to the end point of martensitic transformation M.sub.F, the speed of this cooling being not less than 60.degree. C./s, in order to obtain a structure comprised almost entirely of martensite; PA1 d) The wire is then heated to a temperature, referred to as the annealing temperature, of not less than 250.degree. C. and not more than 700.degree. C., in order to cause the formation for the steel of a precipitation of at least one carbonitride and/or carbide of the alloy element or of at least one alloy component, and the formation of a structure consisting almost entirely of annealed martensite; PA1 e) The wire is then cooled to a temperature of less than 250.degree. C.; PA1 f) The wire is then deformed at a deformation rate .epsilon. of not less than 1.
However, the method described in this document has the following drawbacks:
It is generally known that, in the methods for fabricating martensitic steel pieces, the addition of an alloy element such as vanadium or chromium makes it possible to improve the hardenability and strength following the precipitation of carbonitrides and/or vanadium or chromium carbides during annealing. However, the usual treatment times are several tens of minutes, even several hours, so as to permit precipitation.